The complex bacterial masses which adhere firmly to teeth, dental plaque, provide most of the causes of dental caries and periodontal disease. We are only beginning to understand the variety of molecules, their nature and their interactions which are responsible for the adherence of plaque. A thorough understanding of these molecular interactions should lead to innovations for the control of plaque, and thereby to the control of dental caries and periodontal diseases. Among the important interactions are numerous examples of specific coaggregation between bacteria of different species, e.g., Streptococcus sanguis and Actinomyces viscosus. S. sanguis is often the most abundant bacterium identified in the early colonization of freshly cleaned teeth in man. Actinomyces, which are implicated in root caries and periodontal diseases, increase in number rapidly during the first several days of plaque formation. The coaggregation between these two may be important in the build-up of the Actinomyces population. The objective of this proposal is to provide an understanding of the molecular basis of coaggregation between S. sanguis 34 and A. viscosus T14V as a first model, and then other coaggregating pairs of plaque bacteria. We have isolated a crude water-soluble material from S. sanguis 34 which, in very small amounts, will inhibit the coaggregation. We plan to isolate the coaggregation molecules in highly pure form and determine the structural features of the specific reactive sites.